1. Field of the Invention
The present invention relates to a synchronous machine connected to an alternating current electric power transmitting or distributing system, and more specifically to an alternating current synchronous machine which facilitates absorbing specific higher harmonics superimposed on a fundamental wave voltage of a connected alternating current system (AC system).
2. Description of the Prior Art
Synchronous machines are widely used for generators in power stations, constant speed motors, etc. Therefore, the synchronous machines are in most cases directly connected to AC systems.
On the other hand, also connected to the AC systems are a number of power converters and rectifiers provided with electric power thyristors, etc. which may cause higher harmonics. These higher harmonics have the frequency n times higher than the fundamental AC wave voltage of the system, are harmful to electric devices connected to the system, and can invite malfunctions in communications devices in the vicinity. Under such conditions, there is the serious problem that these higher harmonics should be successfully removed from the AC systems. The above described synchronous machine is provided with a damper winding to absorb higher harmonics generated in the AC system. That is, the damper winding is provided on the pole face of the magnetic pole of the synchronous machine. Practically, plural slots are made on the magnetic pole and filled with damper bars to form a cage type conductor by short-circuiting both ends of the damper bars. This configuration is normally used to reduce the fluctuation of the internal phase angle caused by a sudden change of load. Additionally, it has the function of absorbing higher harmonics by supplying an induced current which reduces a magnetic field generated on an armature by a higher harmonic current. That is, if a higher harmonic current flows through the armature of a synchronous machine from the AC system connected to the armature, then a higher harmonic magnetic field is generated asynchronous with the fundamental AC wave. This magnetic field generates an induced current through the damper winding, which reduces the higher harmonic magnetic field, and absorbs the higher harmonic current.
As described above, the higher harmonics are absorbed by the damper winding by causing the induced current through the damper winding to reduce the higher harmonic magnetic field generated on an armature winding. The damper winding, which is composed of a damper bar and a short-circuit conductor, generates a large leakage inductance, which allows insufficient induced current. Therefore, the asynchronous higher harmonic magnetic field generated by the higher harmonic current from the AC system cannot be completely reduced. As a result, the above described damper winding has failed in sufficiently reducing higher harmonics.